Summary

The shift from herbaceous (grass and sedge dominated) wetlands to swamp forest occurs when wetlands predominately covered in herbaceous vegetation become invaded by woody plant species and irreversibly change to a forest state. This shift occurs when wetlands experience disturbances that affect their hydrology and their natural disturbance regimes, notably their fire regime. In this case study in Zululand, large scale afforestation in the landscape surrounding the wetlands has changed the catchment from a system that uses a low amount of water to a system that uses large amounts of water (through different transpiration rates). This drying has led to shorter periods of soil saturation, which has altered the hydrology of the wetlands. Together with fire suppression, this has caused a shift in regime from herbaceous wetlands to wetlands dominated by indigenous swamp forest species. This regime shift has impacted biodiversity as the wetlands support a rich biodiversity of herbaceous species including the only known wild population of the critically endangered Kniphofia leucocephala. These wetlands are within one of the key water source areas of South Africa, thus this change has affected water supply. Local communities use these wetlands for grazing cattle, and this shift has reduced the area available for grazing. Removing trees from the wetlands is often difficult, therefore managerial recommendations focus on the avoidance through the establishment of a frequent fire regime (biennial) to prevent the recruitment of young trees.

Human Well-being

Key Attributes

Spatial scale of RS

Time scale of RS

Reversibility

Evidence

Confidence: Existence of RS

Contested – Reasonable evidence both for and against the existence of RS

Confidence: Mechanism underlying RS

Well established – Wide agreement on the underlying mechanism

Alternate regimes

Herbaceous Wetland

This regime consists of hygrophilous grasslands and wetlands. These hygrophilous systems vary depending on soil saturation and depending on the level of wetness (temporarily, seasonally or permanently wet); different vegetation types will therefore be found in different wetlands. In areas with low levels of wetness, hygrophilous grasslands (occurring in temporary wet areas) will be found, while areas with increased levels of wetness have typical herbaceous wetland species such as Cyperus species and Nymphaea species, which occur in seasonally to permanently wet areas. Depending on the level of soil saturation, water may be visible depending on the season. Historically, some of these wetlands had so much water that they sustained fish.

Swamp Forest/ Macaranga capensis and Staenoclina tenuifolia woodland

This regime consists mainly of indigenous swamp forest species, notably Ficus trichopoda, Bridelia micrantha, Syzygium cordatum, Staenoclina tenuifolia and Macaranga capensis. Most areas become dominated by mainly Macaranga capensis and the fern Staenoclina tenuifolia, which establishes a dense under-storey, to the exclusion of any other herbaceous plants.

Drivers and causes of the regime shift

The main drivers of this change are plantation forestry in the surrounding landscape and fire suppression. Plantation forestry leads to large scale drying of the landscape (which affects hydrology of the wetlands), and in some cases the wetlands themselves have been converted into plantation forest. With less water available for the wetlands due to the high water interception and transpiration rates by plantation forests (Le Maitre et al. 1999), levels of soil saturation have decreased. At the same time, fire has typically been suppressed in the wetlands and adjoining areas to avoid losses to plantation forests. These factors have allowed forest species such as the fern Staenoclina tenuifolia and Macaranga capensis to invade. Over time, the combination of fire suppression and drying encourages the establishment of woody seedlings, turning wetlands into swamp forests and woodlands. This regime shift is more evident in wetlands which were once planted with plantation forest species with no sufficient woody plant species control to accompany the withdrawal of plantation from these wetlands.

How the regime shift worked

Herbaceous wetlands consisting of hydrophilic grass and sedges are normally surrounded by grasslands. According to Henkel et al. (1936) the Kwambonambi area experienced regular fires, which was noted to be of great ecological importance in hindering the expansion of forest which was confined to locations protected from fire. In wetlands, hydrology is the main driver of vegetation structure, and wetlands with high levels of wetness naturally keep tree densities low. However, wetlands which are temporary to seasonally wet require fire to minimize tree densities. Therefore, high levels of wetness and regular fire in seasonally and temporarily wet wetlands were what maintained the herbaceous regime.

Large-scale plantations have important hydrological effects, including runoff reduction and increased water table fluctuations, as well as ecological effects such as drying out of wetlands and the destruction of natural habitats (Helmschrot 2005). These effects have resulted in drying of the wetlands past their point of historic desiccation. This, in turn, has encouraged the establishment of woody seedlings in the wetlands. Combined with fire suppression this has pushed the system past the ecological threshold to maintain the herbaceous vegetation state. Woody seedlings are typically killed by fire, and in the past frequent fires would have prevented the seedlings from reaching a height where they are immune to fire. During periods of extended fire suppression, the seedlings are able to grow large enough to be able to survive a fire when it finally occurs. In addition, the swamp forest species have a low flammability and impede the spread of fire, so that once established fires becomes less frequent, further encouraging the establishment of woody seedlings, and further outcompeting the herbaceous vegetation for space and light. More generally it has been found that vegetation patterns shift spatially during decade-long dry periods with trees encroaching into herbaceous wetlands, with the frequency and severity of the fire determining the degree of shift (Kirkman 1995).

The forest regime typically begins with fern species, particularily Staenoclina tenuifolia, encroaching into the wetlands and forming dense mats. Adie et al. (2011) showed that the presence of Pteridium aquilinum (bracken fern) aided in the transition of grassland to forest in periods of long fire suppression by providing establishment opportunities for resprouting early-successional forest species. Bracken fern is described as a landscape transformer which plays a pivotal role in montane grasslands by directing plant community development down alternative, and irreversible, successional pathways from grassland to forest (Adie et al. 2011). During periods of long fire suppression, the ferns in this case study, Staenoclina tenuifolia, appears to play a similar role in aiding the development of forest which they thrive under, growing to heights above a meter high. However, if management applies frequent burns, ferns would aid in increasing the fire intensity of burns, as ferns were found to burn at higher temperatures than the grass (Adie et al. 2011). Once a swamp forest is formed, fire becomes infrequent due to swamp forest species having low flammability, providing further establishment opportunities for tree seedlings.

Impacts on ecosystem services and human well-being

Wetlands play an essential role in the water cycle and in access to water quantity and quality. In this case study, the wetlands drain into the two lakes in this region (Lake Mzingazi and Lake Nhlabane), which are important for the local municipality’s water supply. This water recharge ability has been greatly diminished with the shift to swamp forests in many of the wetlands. Evapotranspirative loss from forested wetland is markedly higher than from grass/sedge wetland, resulting in lower water availability for downstream ecosystems and human use. Clulow et al. (2012) consistently measured much higher levels of evaporation in the swamp forest than in the sedge and grass wetland types. The regime shift has also contributed to the loss of grazing land, putting added strain on the wetlands which have not undergone a regime shift. Wetlands often have high biodiversity value, and these wetlands are no different. These wetlands are habitat to the only known wild population of the critically endangered Kniphofia leucocephala. The wetlands are also part of an endangered ecosystem type, hence the significance of conserving these wetlands. This regime shift has been favourable to the plantation companies as forests pose fewer fire risks than grasslands and herbaceous wetlands as they are less flammable, and it saves them money which they would have to pay for fire fighting labour, water and fuel. The diminished ability to recharge the lakes affects industry downstream, particularly the mining industry that utilizes large amounts of water, and municipal water supply. Although not a major concern presently given the high rainfall of the area, it could be a limiting factor in the future. With climate change, competition for water resources will intensify and the scarcity of water will increase the value of wetlands as the population increases.

Management options

Fire is the main reason why some of the herbaceous wetlands in this region have maintained their ecological structure and integrity. Burning the wetlands biennially, during the dry season, has been able to enhance the resilience of these wetlands to invasion by swamp forest species. Commonly, the lower the level of wetness, the more readily woody plants are able to invade a wetland area. This highlights the need for an active fire regime particularly as the wetlands are becoming drier. Another intervention would be to expand the buffer around the wetlands that have maintained their integrity. This is expected to increase the amount of water in the wetlands which will improve wetland hydrology and ultimately improve their resilience.

Reversing the forest regime back to herbaceous wetlands requires mechanical clearing and active fire to enhance and maintain the integrity of the wetlands. Removal of hardwood forest species from wetlands can restore the herbaceous vegetation-fire feedback mechanism and ultimately restore the system to an herbaceous state (Martin & Kirkman 2000). The ferns in this region contribute to high fire intensities aiding in the mortality of trees during a fire event. However, clearing established forest is typically a very expensive endeavour. It is therefore not impossible to try and restore these wetlands back to their herbaceous regimes, but would require significant resource investments.

Helmschrot J. 2005. Assessment of temporal and spatial effects of land use changes on wetland hydrology: A case study from South Africa. Wetlands: Monitoring, Modelling and Management. Taylor & Francis, London.